1. Field of the Invention
The present invention relates to a memory interface circuit for a display device and an access method of the memory interface circuit. More particularly, the invention relates to a memory interface circuit and an access method for converting a single-scan data signal which is suitable for displays such as CRTs or active-matrix liquid crystal displays, into a dual-scan data signal which is suitable for fast-responding passive-matrix liquid crystal displays.
2. Description of the Related Art
The two primary types of liquid crystal displays (LCDs) which have evolved over the years have been active-matrix LCDs and passive-matrix LCDs. Active-matrix LCDs utilize, as switching elements, thin film transistors (TFTs) or other active devices such as metal-insulator-metal (MIM) elements. The switching elements are disposed at every pixel location, i.e., at every crossing of scanning signal lines (rows) and data signal lines (columns). The liquid crystal material in an active-matrix LCD is commonly driven in a twisted nematic (TN) mode, and each switching element is used to apply a constant voltage directly and independently to the corresponding pixel. The result is fast pixel response and excellent contrast.
TFTs are fabricated by depositing and patterning a semiconductor material on a substrate through a series of complex photolithography processes. This results in low fabrication yields and corresponding high costs associated with active-matrix displays. Consequently, it is particularly difficult to realize high definition active-matrix LCDs having a large size with a low cost.
In passive-matrix LCDs, the liquid crystal material is commonly driven in a supertwisted nematic (STN) mode. Since the passive-matrix LCDs do not require active switching elements at every pixel, passive-matrix LCDs are relatively easy to manufacture so that large-sized display panels can be realized at a considerably lower cost than that of active-matrix LCDs.
In a conventional STN passive-matrix LCD, the pixels are addressed in a sequential row-by-row manner. Initially, a large voltage pulse is applied to the first row while a zero voltage is applied to all the other rows. An additional voltage for display is applied to each column corresponding to each pixel in the thus selected first row which is to be turned on. The voltage applied to the first row is then turned to zero and a large voltage pulse is applied to the second row which is next to be selected. In this way, the entire display is scanned one row at a time before again selecting the first row. This type of driving method is referred to as duty driving. In duty driving, a selected pixel receives a single selection pulse having a relatively high voltage level once a frame period.
This straightforward row-by-row selection or addressing has been effective in conjunction with slow-response LC material since the LC material itself tends to average the effect of the applied effect across many frames. The response of the LC material is too slow to react to the instantaneous pulse applied during a single row-selection time. Thus, the LC material responds to an effective value of the applied voltage and the optical state of the pixel is determined by the root-mean-square (rms) value of the applied voltage.
Suppose the effective voltage value applied to a selected pixel and that applied to a non-selected pixel is represented by V.sub.on(rms) and V.sub.off(rms), respectively. The maximum value of a selection ratio V.sub.on(rms) /V.sub.off(rms) is given by the following expression: ##EQU1## where N is a number of rows (scanning signal lines), and 1/N is a duty number. The voltage V.sub.off(rms) is usually set at a threshold voltage V.sub.th of the LC material. As long as the LC material responds to the rms value of the applied voltage, an adequate contrast can be obtained in the display.
However, in order to use passive-matrix LCDs for television, computer monitors, video games, and the like, the passive-matrix LCDs are required to respond quickly enough for displaying video images and mouse use, as well as provide high resolution. It is not difficult to make a fast-responding LCD panel by using a thinner cell gap or a low-viscosity LC mixture so as to reduce a characteristic time constant of the LCD. However, these changes make the liquid crystal material in the pixels respond to large pulses of a voltage signal used for addressing but not to the rms value of the applied voltage signal. This degrades the contrast of the LCDs when addressed by the above mentioned sequential row-by-row scanning. As the characteristic time constant of the LCD approaches the frame time, in a selected pixel, a transmission state of the LC material which is allowed by the selection pulse of the voltage signal cannot be held for one frame period although a sufficient rms voltage value V.sub.on(rms) is applied to the pixel. This reduces an on-state transmittance of the selected pixel. Similarly, in a non-selected pixel, an off-state transmittance is increased although a rms voltage value V.sub.off(rms) is set at V.sub.th. This results in flicker, low contrast, display noise, and the like, and is referred to as "frame response".
Accordingly, in order to obtain high contrast of fast-responding STN-LCDs with high resolution, this "frame response" effect should be counteracted by some method. For this purpose, an addressing technique has been proposed in which a plurality of scanning signal lines are selected simultaneously, and each row (scanning signal line) is supplied with a plurality of relatively small selection pulses per frame. This technique is referred to as "active driving" compared with the above mentioned duty driving. Active driving utilizes a cumulative response of the LC material so as to realize high contrast and fast response.
In passive-matrix LCDs, each pixel cannot be directly driven. Thus, if a plurality of scanning signal lines are selected simultaneously, a display data signal for pixels on a selected scanning signal line interferes with the display data signal for pixels on other selected scanning signal lines via the corresponding data signal lines. Therefore, in order to drive passive-matrix LCDs using the active driving technique, an input video data signal is required to be transformed by an orthogonal matrix prior to being applied to the data signal lines. Based on such transformation, the original input video data is reproduced on the display by multi-line selection of the scanning signal lines.
FIG. 1 shows an conventional LCD device 100 which utilizes an active driving method. As shown in FIG. 1, the LCD device 100 includes a fast-responding LCD panel 101, a segment driver (data driver) 102 for driving data electrodes (not shown) of the LCD panel 101, and a common driver (scanning driver) 103 for driving scanning electrodes (not shown) of the LCD panel 101. In addition, the LCD device 100 includes an orthogonal function ROM 104 for storing orthogonal functions, and an orthogonal transformer 105 for performing an orthogonal transform of an input video data signal according to the orthogonal functions stored in the orthogonal function ROM 104.
The orthogonal transformer 105 transforms the input video data signal using an orthogonal matrix provided by the orthogonal function ROM 104. The transformed video data is applied to the data electrodes via the segment driver 102 as data signals. Column vector elements of the orthogonal matrix are applied via the common driver 103 to the scanning electrodes as pulse scanning signals. Thus, an inverse orthogonal transform is performed on the transformed video data on the LCD panel 101 so that the input video data is reproduced on the LCD panel 101.
The above described active driving technique is categorized into two methods, namely active addressing (AA) and multiple line selection (MLS). The AA method is described for example, in T. J. Scheffer, et. al., SID '92 Digest, pp. 228-231, and Japanese Laid Open Patent No. 5-100642 (corresponding U.S. Pat. No. 5,420,604). In the AA driving method, all the scanning electrodes are selected at a time. The scanning signals, which are generated by ortho-normal functions such as Walsh functions, are applied to the all scanning electrodes. The scanning signals have two voltage levels (positive and negative).
The MLS method is described, for example, in T. N. Ruchmongathan et al., Japan Display, '92 Digest, p. 65, T. N. Ruchmongathan, Japan Display '92, pp. 77-80, and Japanese Laid Open Patent No. 5-46127. In the MLS method, one frame period is equally divided into a plurality of subperiods, and scanning electrodes are divided into subgroups. A different subgroup of scanning electrodes is selected every subperiod, so that every scanning electrode is selected once a frame period by subgroup.
The number of the scanning electrodes which are selected simultaneously in the MLS method is smaller than that in the AA method, so that the MLS method has an advantage of decreasing the size of a computation circuit for orthogonal functions for performing orthogonal transforms. However, the MLS method needs a three-value driver for the scanning electrodes, since the scanning signals are required to have two selected voltage levels (positive and negative) and a non-selected voltage level (zero).
In the MLS method, in the case where a selection number n of the scanning electrodes which are included in each subgroup (i.e., the number of the scanning electrodes which are selected simultaneously) is relatively small, a multi-level driver which has n+1 output voltage levels is required for driving the data electrodes. In the MLS method in which the selection number n is relatively large and in the AA method, an analog output driver is required for driving the data electrodes since the load of the data electrodes increases.
The MLS method includes two types, dispersion type and non-dispersion type, with respect to how to select an orthogonal function matrix. FIGS. 2A to 2C show examples of orthogonal function matrices for the AA method, the dispersion type MLS method, and the non-dispersion type MLS method, respectively. In the dispersion type MLS method, selection pulses are distributed relatively uniformly over one frame in the scanning signals. Thus, in general, the dispersion type MLS method can achieve good contrast by a smaller selection number n of the scanning electrodes as compared to that of the non-dispersion type MLS method. For example, in fast-responding STN-LCD panels having VGA resolution, the selection number n is usually set in a range of 7 to 15 for the dispersion type MLS method, while n is set in the range of 60 to 120 for the non-dispersion type MLS method.
In order to perform an orthogonal transform for the input video data signal, n elements of a display video data vector in a column direction and corresponding elements in a column vector of the orthogonal function matrix are multiplied and summed together. Therefore, the video data is required to be scanned in the column direction on a display screen in the active driving method. Since conventional video data for televisions, personal computer monitors, and the like is scanned in the row direction of the display screen, means for storing the video data signal such as a frame memory is required in order to rearrange the video data signal so as to be correctly displayed on the screen of the LCD utilizing the active driving method.
The capacity of a frame memory depends on the structure of the orthogonal function matrix, i.e., the operation procedure in a frame period. In the AA method and the dispersion MLS method, the selection pulses of the scanning signals are distributed over one frame period, so that the frame memory should have a capacity for storing the video data signal for one frame.
In addition, in order that the inverse orthogonal transform be performed on the LCD panel so as to display the original video data of the current frame, the video data stored in the frame memory should not be changed for one frame period, since the orthogonal transform and the corresponding inverse transform are completed by respective processes through one frame period. Therefore, during the time when the stored video data of one frame is being read out from the frame memory and displayed on the LCD panel, the input video data of the next frame should be written in another frame memory. In this manner, the input video data signal is continuously provided to the LCD panel for each frame period. Therefore, the memory means is required to have a capacity for storing the video data signal for two frames. For example, the memory means has one memory part for storing one frame and another for storing the other frame. This allows a double-buffer operation for the memory means, in which a write (store) operation and a read operation are performed alternately for the two memory parts.
In the non-dispersion MLS method, as understood from the orthogonal function matrix shown in FIG. 2C, the orthogonal transform operation is sequentially performed block by block. The number of blocks is given by dividing the total number of scanning electrodes by the selection number n. Thus, a memory of the LCD utilizing the non-dispersion MLS method is required to store the input video data signal for one block instead of one frame. This makes it possible to reduce a size of the memory. Of course, in order to perform the double-buffer operation for the block data signal, the memory must be capable of storing the input video data signal for two blocks.
The selection ratio of the LC material expressed in Eq. (1) decreases sharply as the number N of the scanning electrodes (scanning lines) increases. For example, in the case where N=240, the selection ratio is about 7%; in the case where N=480, the selection ratio is about 5%. The decrease in the selection ratio causes crosstalk between the scanning signals and the data signals, resulting in degradation of the display quality.
In order to avoid such a decrease of the selection ratio, especially in LCDs having on the order of hundreds of scanning electrodes or more, a LCD panel is divided into two parts each having a half of the scanning electrodes. Each part of the LCD panel is driven independently, so as to gain a higher selection ratio and maintain a large display size and appearance. This kind of driving method in which a display panel is divided into two parts (upper and lower) and each part is scanned independently in one frame period is referred to as "dual-scan driving". A conventional driving method in which one display panel is sequentially scanned from the top to the bottom in one frame period, such as that used for CRTs, is referred to as "single-scan driving".
The selection ratio of fast-responding STN-LCDs which utilize active driving such as the MLS method is the same as that of LCDs which utilize sequential row-by-row driving, so that the selection ratio also depends on the total number N of the scanning lines. Thus, most fast-responding STN-LCDs are driven using the dual-scan driving method. In order to display conventional video data signals for single-scan display systems such as CRTs using dual-scan display systems such as LCDs, the single-scan data signal is required to be converted into a dual-scan data signal which is suitable for display in the dual-scan display system.
This single-scan/dual-scan conversion can be performed, for example, using a memory buffer having two memory regions so as to store the input video data signal for two frames. Each memory region has portions corresponding to the upper and lower halves of the LCD screen. By alternately writing and reading the input video data in/from the two memory regions, the input video data which is written to the memory buffer in a single-scan manner can be read out and displayed in a dual-scan manner (double-buffer operation for single-scan/dual-scan conversion).
A dual-port memory which can perform random write operation and serial read operation simultaneously may be used as a memory buffer. In the dual-port memory, the input video data can be written to addresses from which the stored video data of the previous frame has been read out. This makes it possible to realize the double-buffer operation using a smaller memory capacity for one frame instead of two frames, though dual-port memories are relatively expensive.
In the active driving method, the input video data is written in a memory buffer in a serial manner in the row direction, as in the case of a conventional driving method. However, in order to perform the orthogonal transform, as discussed above, the input video data stored in the memory buffer has to be read out for the selected scanning electrodes column-by-column by the time unit which is obtained by dividing one frame period by the number of pixels. A timing using such a time unit is referred to as a dot clock.
Therefore, the dual-port memory used for the single-scan/dual-scan conversion, from which the stored data is read out in a serial manner only, cannot be used as a memory buffer for the orthogonal transform operation for active driving.
A general-purpose memory can be commonly used for the single-scan/dual-scan conversion and the orthogonal transform operation. However, in the dispersion type MLS method, the memory capacity is required to store amounts of data twice as large as the video data for a whole display screen. Thus, in the non-dispersion type MLS method, which was advantageous initially for reducing the required memory capacity, the memory buffer is also required to store amounts of data twice as large as the video data for a whole display screen in order to perform the single-scan/dual-scan conversion.
FIG. 3 shows read and write operations of a memory buffer 130 which is implemented by using a general purpose memory and commonly used for the single-scan/dual-scan conversion and the orthogonal transform operation of active driving. As shown in FIG. 3, the memory buffer 130 includes two memory regions 110 and 120, each for storing the video data for one frame. The first memory region 110 is divided into two memory portions 111 and 112 corresponding to the upper and lower halves of the LCD screen, respectively. Similarly, the second memory region 120 is divided into two memory portions 121 and 122 corresponding to the upper and lower halves of the LCD screen, respectively.
The data signal of the input video data for frames A, B, C, D, . . . is supplied to the memory buffer 130 in a serial manner. As shown in FIG. 3, the video data of frame A is written in the first memory region 110 by scanning in the row direction. The data for the upper half and the lower half of the display screen are stored in the respective portions 111 and 112 in a time sequential manner. During the time period when the next video data of frame B is written in the second memory region 120, the stored video data of frame A is read out from the first memory region 110 by scanning in the column direction. That is, the video data for the upper half stored in the portion 111 and that for the lower half stored in the portion 112 are read out simultaneously.
Similarly, the next video data of frame C is written in the first memory region 110 by scanning in the row direction, while the stored video data of frame B is read out from the portions 121 and 122 of the second memory region 120 by scanning in the column direction. Then, the next video data of frame D is written in the second memory region 120 by scanning in the row direction, while the stored video data of frame C is read out from the portions 111 and 112 of the first memory region 110 by scanning in the column direction. Thus, by alternately performing write and read operations for the first and second memory regions 110 and 120, the single-scan input video data signal is converted in a dual-scan video data signal, and is displayed on the LCD panel after being subjected to the orthogonal transform.
As described above, in the conventional method, a memory buffer of a fast-responding STN-LCD is required to have a memory capacity for storing the input video data for two frames, in order to perform the single-scan/dual-scan conversion for displaying the video data carried by a single-scan video signal on a dual-scan type display, and to perform the orthogonal transform for active driving. This requirement is independent of the particular accessing methods: the AA method, the dispersion type MLS method, and the non-dispersion type MLS method.